Obsidian Hydration Analysis Selection Considerations

Larger numbers of analyzed artifacts will produce results that are statistically more significant. The factors that influence the formation of a hydration rim over time are complex and even specimens manufactured at the same time are unlikely to all develop hydration rims of identical dimensions. This is one of the reasons that we recommend that rim values be used primarily as a relative, not an absolute dating method. Larger numbers of measurements are more likely to reveal accurate temporal trends and are less likely to be influenced by outliers. In other words, it’s usually better to analyze more artifacts from fewer units than fewer artifacts from more units.

The placement of the hydration rim sample cut. Different artifact surfaces may be created at different periods of time through breakage, rejuvenation, and scavenging. A technological analysis of the artifact to be analyzed may prove to be a useful and productive part of the research strategy. The careful placement of multiple cuts at surfaces of different potential ages, for example, can provide evidence not only of the original manufacturing event but of later periods of reuse.

We hypothesize that stratigraphically, artifacts tend to migrate upwards over time as archaeological deposits accumulate. During site formation activities, some older artifacts will become mixed with contemporary materials, a cycle that will repeat itself throughout the history of the formation of the site. Because of this, the hydration rim values of artifacts from a buried stratum will span not only the range of the deposition of that particular stratum, but all other underlying units as well. Similarly, the artifacts collected in the uppermost deposits (or on the surface) of a site tend to provide a temporal sampling of the entire period of occupation at the site. In practice, we have observed that a carefully collected sample of artifacts from apparently deep and undisturbed deposits will generally show a surprisingly weak relationship between depth and hydration rim width. The hydration analysis of a simple surface collection of artifacts may prove to be an effective and simple way in which to assay the occupational history of the entire underlying site.